1. Field of the Invention
This invention relates to a night vision surveillance system, and more particularly to a method for controlling a light source in a night vision surveillance system with low light noise.
2. Description of Related Art
A low-light-level (LLL) camera, or called as a light intensified camera intensifies its collected light under low light level condition. Under a dim environment with, for example, moon light or star light, the low-light-level camera can collect and intensify the dim light reflected from an object to form a visible image. The LLL camera therefore achieves the purpose to observe objects during the night. Basically, the LLL camera achieves its purpose by intensifying the collected dim light signal. If the environment is fully dark or light intensity is too low, the LLL camera may lose its function to observe objects. In order to solve this problem of insufficient light intensity, a long range night vision surveillance system with high performance usually includes an active illuminating device, such as a laser light source, to actively provide the necessary light intensity for the LLL camera. The LLL camera therefore is not limited by the dark environment. Generally, the illuminator used in night vision system includes a continuous waveform (CW) active-light illuminator or a pulse waveform (PW) active-light illuminator. The illuminator includes, for example, a laser light source.
FIG. 1 is a drawing, schematically illustrating a conventional CW active-light illuminator used with a low-light-level camera. In FIG. 1, an active illuminator 100, such as a laser light source, emits a light to illuminate an object 120, which reflects the light to a low-light-level camera 110. The active illuminator 100 usually is located at a place near to the low-light-level camera 110 by about less than 30 cm, in which two optical axes of them remain in parallel. The active illuminator 100 emits light with continuous waves. An output power of the active illuminator is Po, the low-light-level camera 110 has a receiving power Prec, and the object 120, or called a target, reflects the incident active light with a power PTarget arriving the camera. The active light emitted from the active illuminator 100 may be scattered by particles in the atmosphere. All lights scattered by the atmosphere into the lens of the low-light-level camera 110 produce a scattering power PBS. The receiving power Prec naturally has a relation:
Prec=PTarget+PBS,
in which the PTarget is the surveillanced signal and the PBS is the light noise. Since the CW active-light illuminator is affected by the light noise from back-scattering, it is limited to apply in the long-range night vision system.
FIG. 2 A is a drawing, schematically illustrating a conventional PW active-light illuminator used with a low-light-level camera. In FIG. 2A, a low-light-level camera 210 is usually gated off. An active illuminator 200, such as a laser source, emits a light pulse with a very short pulse width. As the light pulse is reflected by an object 200 onto the low-light-level camera 210, the low-light-level camera 210 is gated on for a short time to receive the light pulse. After that, the low-light-level camera 210 is gated off again until the second pulse is emitted and reflected back from the target. In this manner, the light noise can be reduced.
FIG. 2B is a timing sequence of operation of the PW illuminator in FIG. 2A. In FIG. 2A and FIG. 2B, the light pulses are periodically emitted as denoted by xe2x80x9cONxe2x80x9d. The xe2x80x9cOFFxe2x80x9d status of the light means that there is no light pulse emitted. The light pulse has a very short width Tp with a shape like the third pulse in FIG. 2B, in which Tp is also the time period for gating on the low-light-level camera 210. A distance xe2x80x9cdxe2x80x9d in FIG. 2A represents the distance between the light source 200 and the object 220. Light travels in the light speed c as usual as a physics phenomenon. In this setup, the low-light-level camera 210 is gated off for a period of T2d=2d/c and then gated on with a period of time Tp, which is denoted as xe2x80x9cONxe2x80x9d at the lower timing sequence. There is an idle period of Ti before the next light pulse being emitted. Ti can be set to zero if possible for the active illuminator 200.
In the manner shown in FIGS. 2A and 2B, the back-scattering lights as shown in curved arrows basically arrive at the low-light-level camera 210 before the light pulse reflected by the object 220 arrives because of the traveling distance. Thus, the back-scattering lights are not xe2x80x9cvisiblexe2x80x9d by the low-light-level camera 210, which is still gated off when the back-scattering lights arrive. Thereby, the light noise is reduced. However, since the light pulse has very short width, each light pulse can only provide a very limited power. In other word, the light system, such as a laser light source, usually has a low duty cycle, which is a ratio of the emitting pulse width to the period of the light pulses. In FIG. 2B, the duty cycle is equal to Tp/(T2d+Ti+Tp). Furthermore, if the observing distance d is very long, the duty cycle basically is inversely proportional to the observing distance d. In order to achieve the illuminating purpose, a peak power of the light source 200 of FIG. 2A needs to be very large. This limits the observing range of the low-light-level camera.
In summary of the conventional active-light illuminators, some drawbacks are as the following:
1. For the conventional CW active-light illuminator, the light is continuously emitted. This causes that the low-light-level camera is continuously opened to receive the reflected light from the object. The light noise from the back-scattering light is inevitably received by the camera, causing a deterioration of the performance. In a practical experience, it can only observe the objects within one kilometer or so under a clear night sky.
2. For the conventional PW active-light illuminator, even though the light noise resulting from the back-scattering light can be avoided by proper ON-OFF timing controls, its duty cycle is strongly reduced, causing a need of a light source with very high peak power. Otherwise, the observing range is also limited.
The observing range of the low-light-level camera with above illuminators is limited due to either the light noise or an insufficient peak power of the light source.
It is at least an objective of the present invention to provide a method for controlling a light source in a night vision surveillance system with low light noise. A larger width of the active light pulse is allowed so that the duty cycle is increased and the ON-OFF timing is also properly controlled so that the observable range of the night vision system is improved without conventional issues.
It is at least another objective of the present invention to provide a method for controlling a light source in a night vision system with low light noise. The light noise from the back-scattering light is effectively filtered away so that a higher contrast of observed objects is achieved.
In accordance with the foregoing and other objectives of the present invention, a method for controlling a light source in a night vision system is provided. The method includes computing a pulse width Tp according to parameters of a desired observing distance d and a shortest distance d, of the back-scattering light that enters the light sensors of a night vision system, such as a low-light-level camera. Accordingly, the duty cycle is obtained. The pulse width Tp=2(dxe2x88x92d1)/c, where c is the light speed. According to the parameters of the desired observing distance and the pulse width, an illuminating control signal and a camera control signal are generated by a pulse signal controller and respectively control a pulsed active-light illuminator and a low-light-level camera with proper timing.
In this manner, the shortest scattering-back distance d1 is set to be about 0.01 to 0.99 of the observing distance d for a range of about 5 kilometers to get a good observed result according to practical experiences. A little adjustment can be performed to achieve an optimized condition. The back-scattering lights are effectively filtered away so that the contrast of the observed object is increased. A peak power of the pulsed active-light illuminator does not require a high power one. The illuminating performance is also improved.